ARM-Darwin: Use the *-*-darwin-eabi triple for v6m & v7m archs

These arch arguments are used for embedded targets (obviously) which need a
different calling convention to iOS.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193328 91177308-0d34-0410-b5e6-96231b3b80d8